A navigation device for a vehicle (for example, an automobile) reads map data including a current position of the vehicle from an external storage medium such as a DVD-ROM or a hard disk drive. The navigation device further displays a map on a display screen based on the map data and displays a mark corresponding to the current position of the vehicle on a fixed position in the display screen (for example, the center of the display screen or a position slightly below the center of the display screen). The navigation device further scrolls the map on the display screen according to travel of the vehicle.
To detect a current position of the vehicle, the navigation device uses autonomous (dead-reckoning) navigation or GPS (Global Positioning System) navigation. The autonomous navigation uses a distance sensor, a gyro sensor, or an earth magnetism sensor; the GPS navigation uses a GPS receiver to receive radio waves from satellites.
In the GPS navigation, a correct position of the vehicle can be detected since the navigation device can calculate an absolute position of the vehicle according to the radio waves from the satellites. However, the navigation device cannot detect the correct position of the vehicle in a place where the GPS receiver cannot receive the radio waves from the satellites, such as a place between tall buildings, below an elevated road, or in an indoor parking area. To cope with the problem, some recent navigation devices use the autonomous navigation in addition to the GPS navigation. More specifically, the navigation device uses the autonomous navigation when the GPS receiver cannot receive the radio waves from the satellites.
Furthermore, autonomous navigation with smaller errors has recently become more required to accurately navigate. This is because places where the GPS receiver cannot receive radio waves from the satellites are increasing. For example, these places are urban roads along tall buildings, three-dimensionally intricate highway entrances/exits/junctions; combinations of a lifted road and ground road extending mutually parallel; underground parking areas; and indoor parking areas.
The autonomous navigation needs to detect a travel distance and a heading direction of the vehicle. The travel distance can be detected by using a distance (speed) sensor installed in the vehicle. The heading direction can be detected by using a gyro sensor (for example, a vibration gyro, a gas rate gyro, a fiber optic gyro) for detecting an angular velocity. When the vehicle travels on a road with a tilt, the tilt causes an error in a travel distance displayed on a two-dimensional map. In addition, a tilt of the vehicle causes a tilt of a detection axis of the gyro sensor, which finally causes an error in the detected heading direction. In Patent Document 1, a navigation device is disclosed which detects the tilt of the road by using a triaxial acceleration sensor and corrects errors in the travel distance and heading direction of the vehicle based on the detected tilt.
Furthermore, in Patent Document 1, to use the autonomous navigation with a high accuracy, the navigation device corrects the travel distance and heading direction of the vehicle by using sensors. An angular velocity sensor is to detect angular velocities in a yaw direction and a pitch direction; an acceleration sensor is to detect accelerations in at least two of a front-rear direction, a lateral direction, and a vertical direction of the vehicle.
This needs to detect a tilt angle (or orientation), relative to the vehicle, of a casing, in which the angular velocity sensor and acceleration sensor are contained. The tilt angle should be detected when the vehicle remains in a horizontal orientation. The orientation of the vehicle is detected by using a lateral vehicle-height sensor and a pendulum type tilt sensor. Thus, this navigation device requires many sensors, which makes the navigation device more complicated.                Patent Document 1: JP 2004-125689A        